1. Field of the Invention
The present invention relates to a plasma processing method for performing film formation (deposition), etching, surface treatment on a substrate and the like, and more particularly, to a plasma processing method for generating plasma based on a plasma processing gas to perform film formation (deposition), etching, surface treatment and the like for a thin film such as a semiconductor thin film, an insulating thin film and so on, and in particular specifying a pressure range of a plasma processing gas in association with a cycle of high-frequency power to thereby enable high speed processing.
2. Description of the Related Art
High rate processing has been demanded in a plasma processing for performing film formation (deposition), etching, surface treatment and the like for a thin film such as semiconductor thin film, an insulating thin film and so on. To get high rate of the processing, a technique for raising pressure of a reactant gas is generally employed. However, when the reactant gas is raised in pressure, it becomes difficult to generate plasma stably.
Japanese Patent Publication No. 60412/1994, Japanese Patent Specification No. 2700177, Japanese Patent Laid-Open No. 299358/1994 and so on disclose a technique for generating plasma stably under high pressure by adding an inert gas to a reactant gas. With these publications, a large amount of inert gas is added to a reactant gas, so that plasma can be maintained stably and the processing speed can be enhanced even when the reactant gas is raised in partial pressure.
The three publications described above are common in a fundamental thought that an inert gas is added to a reactant gas, and plasma processing is performed under high pressure.
Hereupon, contents disclosed in Japanese Patent Specification No. 2700177 will be described as an example with reference to FIG. 4. The publication relates to a thin film forming (deposition) method, of which fundamental thought is common to an etching method, and a surface treatment.
In FIG. 4, the reference numeral 1 designates a reaction vessel. The reference numerals 2, 3 designate opposed electrodes, to either of which high resistance bodies 4, 5 are mounted. The electrode 3 is grounded, and a substrate 6 is loaded on the high resistance body 5 on the electrode 3. The electrode 2 is connected to a high frequency power source 8. Provided laterally of the electrodes 2, 3 are a nozzle 7 and a gas outlet 10 so that a gas is supplied to a gap portion g between the upper high resistance body 4 and the substrate 6. In addition, the substrate 6 is heated by a heater 9 when a thin film is to be formed.
With the above arrangement, a plasma processing gas composed of a mixture gas of a reactant gas and a He gas is introduced into the reaction vessel 1 from the nozzle 7, and a high frequency power is supplied to the electrode 2. When an amorphous thin film Si is to be formed on the substrate 6, a SiH4 gas is used as the reactant gas. Pressure of the plasma processing gas is near the atmospheric pressure, and a percentage of the He gas in the plasma processing gas is 90% or more. The high frequency power has a frequency of 13.56 MHz. The above technique is used to cause glow discharge in the gap g, so that plasma is produced based on the plasma processing gas. Plasma of the SiH4 gas, a reactant gas, is used to form the amorphous thin film Si on the substrate 6. According to the above technique, because the percentage of the He gas is great, glow discharge is generated even under high pressure near the atmospheric pressure, enabling to maintain plasma stably. The above publication describes the following actions as actions of the He gas.
(a) He is liable to be excited by electric discharge.
(b) He has many quasi-stable states, and many active particles (radicals) of He in excited condition can be formed in plasma.
(c) When active particles (radicals) of He are present in high density, dissociation of a reactant gas can be enhanced.
(d) Ion is liable to diffuse in He plasma, and so discharge is liable to spread.
Such properties of the He gas enable maintaining plasma stably even in the condition that the reactant gas is high in partial pressure. Besides, because the reactant gas is high in partial pressure, it is possible to increase a processing rate for plasma processing.
Also, because pressure of the plasma processing gas is near the atmospheric pressure, it is unnecessary to evacuate an interior of the reaction vessel for vacuum. Accordingly, there is no need of a vacuum chamber and a vacuum exhausting apparatus, and so cost required for installation can be much decreased.
With the above prior technique, an inert gas is added to the reactant gas, so that plasma can be maintained stably even when the reactant gas is increased in partial pressure. So, because the reactant gas is high in partial pressure, it is possible to increase a processing rate for plasma processing. In plasma processing in recent years, however, the demand for increasing a processing rate is very strict, as typified by the manufacturing process for amorphous Si solar cells. That is, it is desired to enhance the processing rate further. With respect to improvement of the processing rate, the prior art teaches the following matters.
(1) Japanese Patent Specification No. 2700177
With respect to the case where a plasma processing gas composed of a mixture gas of a reactant gas (SiH4 gas) and an inert gas (He gas) has a pressure near the atmospheric pressure, it is shown that the processing rate is raised by increasing a ratio of the reactant gas to the inert gas. By the way, increasing the ratio of the reactant gas under a condition that the plasma processing gas (mixture gas) is constant in pressure corresponds to increasing the reactant gas in partial pressure.
(2) Japanese Patent Laid-Open No. 299358/1994
It is shown that a processing rate is increased by raising a reactant gas in partial pressure (not less than 10 Torr) and correspondingly adding an inert gas. It is prescribed that pressure of a plasma processing gas is 10.2 Torr to 100 Torr, and such pressure range indicates a limit for generation of plasma with only a reactant gas, and a limit for low temperature plasma. That is, a pressure range determined in terms of increasing the processing rate is not shown.
(3) Japanese Patent Publication No. 60412/1994
A ratio of an inert gas in a plasma processing gas (mixture gas) is prescribed in terms of stability on discharge under the atmospheric pressure, but nothing is shown with respect to increasing a processing rate.
As described above, the prior arts show improvement of a processing rate by increasing a reactant gas in partial pressure. However, the publications do not present any guideline for increasing the processing rate for a constant partial pressure of the reactant gas. That is, the publications do not teach the matter as to “under a condition that the reactant gas is constant in partial pressure, how much inert gas should be added, and what pressure of a plasma processing gas (mixture gas) should be set so as to be effective in increasing the processing rate”.
Meanwhile, to aim at greatly increasing the processing rate, it is very important to enhance an efficiency of use of a reactant gas for a constant partial pressure of the reactant gas to thereby increase the processing rate.
According to the disclosure of Japanese Patent Publication No. 60412/1994 and Japanese Patent Specification No. 2700177, it is unnecessary to evacuate an interior of a reaction vessel, because a plasma processing gas composed of a mixture gas of a reactant gas and an inert gas has a pressure near the atmospheric pressure. Therefore, there is no need of a vacuum chamber and a vacuum exhausting apparatus, and so cost required for installation can be much decreased. However, even if being not evacuated, an interior of the reaction vessel must be filled with a mixture gas of a reactant gas and an inert gas at 1 atmospheric pressure. That is, if the plasma processing gas (mixture gas) is high in pressure, it is necessary to introduce into the reaction vessel a correspondingly large amount of the plasma processing gas (mixture gas). In particular, the He gas used in large amount is expensive, and so a ratio of cost required for the He gas becomes great taking account of cost per device in mass-production factories. According to the above-described prior art, while cost required for installation can be decreased, cost required for consumable goods such as He gas increases, which leads to high cost, judging comprehensively.